Substrate for mounting electronic part and electronic part

ABSTRACT

The present invention is characterized by a structure having a substrate  1 , and metallization layers  2  formed on the substrate  1 , on which a Sn solder film  3  and an Ag film  4  are formed. The Ag film  4  is a metal free from oxidization at room temperature in the atmosphere. In a wet process, since only an exposed side of the Sn solder film  3  is oxidized by the cell reaction of Ag and Sn, an upper surface of the Ag film  4  on the solder film, which would otherwise affect the connection, is not oxidized. Since the Ag film  4  melts into the Sn solder simultaneously with melting of the Sn solder film  3 , the Ag film  4  does not hinder the connection.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 11/378,450, filed Mar. 20, 2006, now U.S. Pat. No. 7,511,232 thecontents of which are incorporated herein by reference.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationserial no. 2005-126080, filed on Apr. 25, 2005, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a substrate for mounting an electronicpart (e.g. an electronic device), in which a solder film is formed onmetallization layers, and an electronic part, in which a solder film isformed on a connection point or a lead surface.

Solder is formed on a substrate, an electrode of an electronic part or alead of an electronic part, in very many products. Specifically, thefollowing measures are frequently used in mounting of an electronicpart: (1) forming metallization layers, which is to serve as anelectrode, on a substrate, forming a solder film on the metallizationlayers, and using the solder film for connection to the electronic part;(2) forming metallization layers on an electrode of an electronic part,forming a solder film on the metallization layers, and using the solderfilm for connection to another electronic part; and (3) forming a solderfilm on a lead surface of an electronic part, which includes a lead, andalso melting the solder film to carry out connection at the time ofconnection to a printed circuit board.

Metallization layers of an electrode and a solder film are formedexemplarily by: (1) coating a solder film on a copper foil on a printedcircuit board with the plating method; and (2) forming metallizationlayers on a ceramic substrate and forming a thin solder film on themetallization layers by means of sputtering or vapor deposition. Anelectronic part formed with metallization layers of an electrode and asolder film is structured exemplarily such that a circuit element isformed on a semiconductor wafer and solder bumps are formed on electrodemetallization layers of a connecting portion thereof. An electronicpart, in which a plated layer consisting of Sn or a plated layer of a Snalloy is formed on a lead surface of the electronic part, is an example,in which a solder film is formed on a lead surface of an electronicpart.

A role which a solder film plays on a substrate or an electronic partwill be described below.

In case of beforehand forming a solder film on a substrate, anelectronic part is mounted so that a connecting portion of theelectronic part abuts against the solder film, the semi-product issubjected to reflow to cause melting of the solder film provided on thesubstrate, and the solder wets and spreads over metallization layersprovided at a connecting portion of the electronic part, etc., toachieve connection of the substrate and the electronic part.

In a sub-mount part, in which metallization layers is formed on asubstrate containing ceramic, Si, etc. and a solder film is formed onthe metallization layers by means of the thin film forming technique, anelectronic part such as an optical element is pressed against the solderfilm, heating in a state without flux is effected to melt a thin filmsolder to cause the solder to wet and spread over the metallizationlayers of the electronic part for connection.

In a part, in which solder bumps are formed on an electrodemetallization layers of an electronic part, solder bumps are in manycases formed by melting solder balls and causing the solder to wet andspread over the metallization layers of the electronic part when afterbeing divided by dicing, solder bumps are to be formed before separatinginto chips. In this case, connection is achieved by mounting theelectronic part on a printed circuit board, a ceramic substrate, etc.and make it being subjected to reflow, so as to melt the solder bumpsand cause the solder to wet and spread over the metallization layers ofthe substrate. Also, in recent years, a circuit element is in some casesformed on a Si wafer and a method such as plating is used to form asolder film at the wafer level in a process prior to division by dicing.

For an electronic part including leads, solder paste is printed on asubstrate, the leads of the electronic part are mounted on the paste,and the whole is subjected to reflow so as to melt the solder to connectthe substrate and the leads of the electronic part to each other.

A plated film consisting of Ag or a plated film consisting of Sn isfrequently used for lead frames of an electronic part. The plated filmconsisting of Ag is not oxidized at its surface and so excellent inwettability. While the plated film consisting of Sn is oxidized at itssurface but a part of the oxidized film become broken somewhere and thesolder on a side of a substrate and the Sn plated film melt to unitewith each other so that connection is achieved.

Japanese Patent Laid-open No. Hei 5-190973 discloses a sub-mount forsemiconductor laser, which is an example of the substrate for mountingelectronic part. The sub-mount adopts Ti/Pt/Au as metallization layersand provides a Pt layer and an Au—Sn solder layer in a region, in whicha semiconductor laser is to be mounted. Metallization layers are alsoformed on a back surface of the semiconductor laser and thesemiconductor laser is fixed firmly by melting the Au—Sn solder on thesub-mount to connect it to the metallization layers.

The reason why an Au—Sn solder has been used in this technical field isthat the Au—Sn solder is high in hardness and creep deformation is hardto occur. This is because when the semiconductor laser generates heat atthe time of light emission to be raised in temperature to cause thesolder to undergo creep deformation, the semiconductor laser is shiftedin position and so optical coupling cannot be obtained.

In recent years, a semiconductor laser, etc. by a GaAs semiconductor hasbeen frequently used as a light source for optical recording. Suchsemiconductor laser is affected by residual stress due to solderconnection when the Au—Sn solder is used, resulting in decreasedreliability in some cases. Such residual stress is generated since thereis a difference in coefficient of thermal expansion between asemiconductor laser and a sub-mount when the semiconductor laser and thesub-mount are fixed at the fusing point of the solder and cooled toaround room temperature. In the case where the solder is soft, thesolder is deformed to relax the residual stress. However, in the casewhere the solder is hard, the effect of relaxing the residual stress issmall.

Accordingly, if the Au—Sn solder is used to achieve connection of asemiconductor laser, of which an element is large in total length, arelatively large residual stress is generated in the semiconductor laserto shorten the life of the semiconductor laser in some cases.

In view of such a background, the use of a solder containing Sn, whichis soft, as a main component for mounting of the semiconductor laser hascome under consideration.

However, a solder film containing Sn as a main component and formed on asubstrate, metallization layers of an electronic part, or a lead surfaceof an electronic part is, in many cases, formed at a surface thereofwith an oxide film. This is because a solder ordinarily contains Sn as amain component and Sn is oxidized in the atmosphere.

With a view to surely achieving connection, it is convenient to use fluxto reduce an oxide film present on a surface of the solder film toimprove the connection property sharply. However, in recent years, it isoften not permissible to use flux.

For example, failure in mounting an optical element is caused when fluxresidues are present in a light emitting portion of the optical elementto intercept an optical path. Also, there may be damage to an opticalelement caused by flux itself or an organic solvent used for cleaning offlux residues.

In case of forming a solder film on a printed circuit board or a ceramicsubstrate, in case of forming solder bumps on a side of an electronicpart, and in case of forming a solder film on a lead surface of anelectronic part, respectively, flux has been hitherto used to decreasean adverse influence caused by the oxide film made on a surface of thesolder film. In recent years, however, it has been increasingly promotedto make a connecting portion minute and to make a pitch small, and thusvaporization of a flux component and flow of flux at the time ofconnection may cause positional shift of a minute connecting portion togenerate a short-circuit failure. Also, the material cost, coatingprocess, and a subsequent cleaning process, respectively, of flux itselfinherently constitute factors for an increase in cost, and so it ispreferable to enable achieving connection simply in a fluxless manner.

It is estimated that a connecting portion is increasingly made minutefor lead frames of electronic parts in the future, and solder pasteprinting on a substrate is approaching a limit in minuteness. That is,if connection can be achieved using solder plating on a side of anelectronic part or the like without applying solder paste printing on asubstrate, it is possible to sharply decrease failure in bridge betweenleads. Also, the use of flux possibly shifts a position of an electronicpart, although slightly, due to generation of bubbles of flux at thetime of heating or the like. Conventionally, since an amount of solderused was much, self-alignment caused by surface tension of molten solderprevented the positional shift of the electronic part, which is possiblycaused by bubbles of flux. However, it can be expected that influencesof such bubbles cannot be neglected when the amount of solder used isdecreased as minuteness is promoted in the future. Also, since the useof flux is inherently a cause for an increase in cost in terms ofmaterial cost, flux coating process, and a subsequent cleaning process,a connection process, in which flux in as small in amount as possible isused, is desirable.

Also, a lead frame plated with Sn always involves a problem that aneedle crystal called whisker grows to cause a short-circuit betweenleads. Making a lead pitch minute means that a short-circuit may becaused by even a shorter whisker, and thus, a further strict controlwill be demanded.

SUMMARY OF THE INVENTION

An object to be solved by the invention is to provide an electronic part(e.g. an electronic device), in which a solder film formed on asubstrate or a connecting portion of the electronic part is preventedits surface from being oxidized and connection can be achieved in afluxless manner.

The object can be attained by a substrate for mounting an electronicpart (e.g. an electronic device), comprising a base material,metallization layers formed on the base material, and a Sn solderportion formed on a part of a surface of the metallization layers, inwhich an Ag film is formed on a surface of the Sn solder portion, thesurface being used for mounting the electronic part.

Also, the object can be attained by an electronic part (e.g. anelectronic device) comprising a base material, metallization layersformed on the base material, and a Sn solder portion formed on a part ofa surface of the metallization layers, in which an Ag film is formed ona surface of the Sn solder portion.

Further, the object can be attained by an electronic part (e.g. anelectronic device) comprising a lead frame, a function element mountedon the lead frame, a plurality of bonding wires, the bonding wireconnecting a terminal portion of the function element and a connectingportion of the lead frame to each other, and a resin part that molds thefunction element, the plurality of bonding wires, and a part of the leadframe, in which leads extending outward from the resin part aresubjected to Sn plating, and Ag films are formed on a connecting portionof the leads, which are subjected to Sn plating. The lead extendingoutward from the resin part is e.g. a portion of one of the lead framewhich is exposed from the resin part and plated with tin (Sn). At leasta portion of the tin-plated lead to which an external member to theelectronic part (e.g. a substrate) is plated with silver (Ag).

While the metallization layers described above has a multilayerstructure formed of a plurality of laminated layers each composed of ametal or an alloy, the metallization layers may be sometime replaced bya metallization layer formed of either a metal layer or an alloy layer.Both the metallization layers and the metallization layer are alsodefined as a metallization area formed in a surface of an insulatingmaterial (like a ceramic) or a semiconductor material to metalize anarea in the surface. In other word, the metallization area is formed ofat least one metallization layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described inconjunction with the accompanying drawings, in which:

FIG. 1 is a cross sectional view showing a connecting portion of thesubstrate;

FIG. 2 is a cross sectional view showing the connection portion of thesubstrate according to an embodiment 2;

FIG. 3 is a cross sectional view showing the connection portion of thesubstrate according to an embodiment 3;

FIG. 4 is a partial, cross sectional view showing the connectionportions of the electronic part; and

FIG. 5 is a cross sectional view showing the electronic part.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments for carrying out the invention will be described below withreference to the drawings.

Embodiment 1

A substrate according to a first embodiment of the invention will bedescribed below with reference to FIG. 1. FIG. 1 is a cross sectionalview showing a connecting portion of the substrate.

The substrate 10 shown in FIG. 1 is provided by Electron Beam depositingmetallization layers (in other words, the metallization area) 2, made ofTi/Pt/Au (deposition in the order of Ti→Pt→Au) at a time on a ceramicsubstrate 1, then forming a pattern by means of ion milling,subsequently forming a pattern with a resist, and vacuum evaporating aSn (tin) solder film 3 and an Ag (silver) surface antioxidization film 4at a time on the semi-product to carry out lift-off. The Ti (titanium)layer included in the metallization layers 2 severs for close contactwith the ceramic substrate 1, the Pt (platinum) layer serves as a solderbarrier layer, and the Au (gold) layer serves to ensure the wire bondingproperty. The metallization layers 2 have a thickness of 0.1 μm/0.2μm/0.2 μm in order from the side of the ceramic substrate 1. Also, theSn solder film 3 has a thickness of 3 μm, and the Ag film 4 has athickness of 0.1 μm.

The Sn solder film 3 serves as a binding agent when the whole substrateis heated to connect electronic parts, etc. thereto. Since the Sn solderfilm 3 possesses the property of oxidation in the atmosphere, however,the Ag film 4 prevents oxidation of the Sn solder film 3. Portions, onwhich the Ag film 4 and the Sn solder film 3 are formed, defineportions, on which electronic parts are to be mounted. Wire bonding isapplied to the portion which the metallization layers 2 are exposed.

The effect of antioxidization of Sn produced by Ag will be describedbelow in detail. Oxidation reaction of Ag is represented by thefollowing formula (1).4Ag+O₂→2Ag₂O  (1)Temperature and oxygen partial pressure where the reaction formulaproceeds rightward can be calculated from free energy of Gibbs of thereaction.

Free energy ΔG of Gibbs is represented as follows by the use of enthalpyΔH, entropy ΔS, and absolute temperature T.ΔG=ΔH−TΔS  (2)According to MATERIALS THERMOCHEMISTRY Sixth Edition (page 258) writtenby Kubacheswski, ΔH⁰=31.1 kJ/mol and ΔS⁰=120.9 J/K/mol in the standardstate of 25° C. are deduced. Thus, a standard free energy of formationof Ag₂O in the standard state of 25° C. becomesΔG ⁰=31.1−298×0.1209 kJ/mol=−4.9282 kJ/molBased on the calculation, a boundary of oxygen partial pressure forformation/decomposition of Ag₂O, that is, dissociation pressure isfound.ΔG=ΔG ⁰ +RT ln K  (3)K=a _((Ag2O))/(a ² _((Ag)) ×PO₂)  (4)wherein R indicates a gas constant, K indicates an equilibrium constantrepresented by the formula (3), a indicates each activity, and PO₂indicates oxygen partial pressure. With the dissociation pressure ofAg₂O, ΔG=0 on the left side of the formula (3), and a_((Ag2O))=1 and a²_((Ag))=1 in the formula (4) presentΔG ⁰ =RT ln PO₂  (5)Accordingly, using ΔG⁰ found previously, the gas constant R=8.314kJ/K/mol, and temperature 298 K (25° C.), PO₂ is calculated as followsby modifying the formula (5).PO₂=ext(ΔG ⁰ /R/T)=0.998 atm.=1011 hPa  (6)Since oxygen concentration at 1 atm. is 21%, oxygen partial pressurepresents 213 hPa (0.21 atm.), which is smaller than the dissociationpressure of Ag₂O. Accordingly, it can be concluded that Ag is notoxidized at 25° C. in the atmosphere. It is found from the above thatsince Ag is not oxidized at room temperature in the atmosphere, theforming of the Ag film 4 on the Sn solder film 3 as the structure shownin FIG. 1 has the effect of preventing oxidization of the Sn solderfilm.

According to the embodiment, it is possible to prevent formation of anyoxide film over a substantially whole region in a wet process of theelectronic part 10. This will be described below.

With the structure shown in FIG. 1, Sn and Ag being nobler than Sncontact with each other and Sn is exposed at side faces. In the casewhere corrosion proceeds due to water in the wet process, Sn being abase metal and exposed at side faces is one-sidedly ionized by aso-called cell reaction. In other words, with a cell, in which Ag and Snmake electrodes and water serves as an electrolyte, the Ag electrode isnot subjected to corrosion as long as the Sn electrode being less nobleremains. That is, the side faces are only partially subjected tocorrosion (oxidization), and a surface of the Ag film 4, which occupiesalmost the whole of the connection surface, is not subjected tooxidization. Thus, in the case where electronic parts are to beconnected by melting the Sn solder film 3, an oxide film that hindersconnection is present only partially on the side faces and has littleadverse influences. In case of taking account of both oxidization byordinary oxygen and oxidization by water and the joint property bysolder, that structure, in which an upper surface of the Sn solder film3 is covered by the Ag film 4 as shown in FIG. 1 and the side faces ofthe Sn solder film 3 are exposed, is preferred from the above. If sidefaces of the Sn solder film are also covered by the Ag film, it can beanticipated that the anticorrosion effect of Ag by the cell reaction inthe wet process is decreased, while the effect of antioxidization isobtained as compared with the case in which the upper surface of the Snfilm is exposed, because Ag is hard to be susceptible to corrosion ascompared with Sn.

When the Ag film is formed on the Sn solder film, there is a possibilitythat an Ag₃Sn compound is formed at a Sn/Ag interface. However, the Agfilm is preserved at a surface of the Ag film over a long term. This isbecause the speed of the diffusion of Ag into Sn is slow. Accordingly,the structure shown in FIG. 1 is preserved over a long term, so that theeffect of antioxidization is also preserved over a long term.

An Ag film having a thickness of 0.1 μm was formed on a Sn solder filmhaving a thickness of 3 μm, so as to confirm the melting property andthe connection property. Consequently, it could be confirmed that the Agfilm melted into a Sn solder simultaneously with melting of the Snsolder film and the Ag film did not have any adverse influence on thewettability and the connection property of the solder.

A film parameter of the Sn solder film and the Ag film is prescribed interms of connection property. That is, an alloy of Sn and Ag has an Agconcentration of 73 wt % and the whole alloy makes Ag₃Sn. With less Ag,a liquid phase component surely remains at a Sn—Ag eutectic temperatureof 221° C. or higher, so that connection is made possible. Accordingly,thicknesses of the Sn solder film 3 and the Ag film 4 should bedetermined so that the Ag concentration at the time of melting becomesequal to or less than 73 wt %.

For example, since silver eating utensils become blackish, it isgenerally thought that silver is liable to oxidize as compared withgold. Since Ag is not susceptible to surface oxidization by oxygen, itis thought that blackening of silver is attributable to oxidization dueto moisture in the atmosphere. However, substrates and electronic partsare ordinarily manufactured in a clean room and kept in a desiccator, inwhich humidity is maintained very low. Accordingly, influences ofmoisture in the atmosphere are negligible.

As described above, it is possible according to the embodiment toprovide a substrate, in which surface oxidization due to oxygen andoxidization due to moisture can be prevented over a major part of aconnection surface and which has a solder film (here, referred to assolder film including Ag) having an excellent connection property. Thestructure according to the embodiment uses Ag for antioxidization and Agis a most inexpensive metal free of oxidization in the atmosphere, sothat an advantage in terms of cost is given.

In addition, while ceramic was used as a substrate in the embodimentdescribed above, a glass substrate, a glass epoxy substrate, asemiconductor substrate, etc. can also be used. While Ti/Pt/Au was usedas the metallization layers in the embodiment, it is not limitative but,for example, Cr/Cu/Au/, Ti/Ni/Au, etc. can also be used. While themetallization area is formed of the metallization layers 2 thuslaminated on the substrate (i.e. a multilayer structure), themetallization area may possibly be replaced by a single metallizationlayer (e.g. of Ni (Nickel)-Cu (Copper) alloy or of Ti (Titanium)) inaccordance with the material of the substrate 1. Also, while a Sn solderwas used as a solder in the embodiment, it is not limitative but analloy solder such as a Sn—Ag solder, a Sn—Ag—Cu solder, a Sn—Zn solder,a Sn—Pb solder, having Sn as a main component (the component having thelargest amount) can also be used. A solder including a Sn solder and analloy solder containing Sn as a main component is referred to as Snsolder. Ag is not limited to pure silver but includes an alloycontaining Ag as a main component. A deposition method is not limited tovapor deposition but may adopt a thin film forming technique such assputtering. In addition, the modifications described above are alsocommon to other embodiments described below.

Embodiment 2

A further embodiment for the substrate according to the first embodimentof the invention will be described with reference to FIG. 2. Here, FIG.2 is a cross sectional view showing a connecting portion of thesubstrate.

The substrate 20 shown in FIG. 2 is formed such that an Au film 60 isformed on the Ag film 4 of the substrate 10 described in theembodiment 1. The substrate is different from that in the embodiment 1in that a Sn solder film 3, the Ag film 4 and an Au film 60 arecontiguously formed on a lift-off resist having a pattern formed, byvacuum evaporation.

Since Au is a metal free of oxidization, it is frequently used for asurface coating film on metallization layers, wire bonding, or the like.Au is sometimes used as an antioxidization film in soldering. However,when an Au film is formed directly on a solder film containing Sn as amain component, Au diffuses into Sn and an antioxidization effect by Auis produced for a short period of time in some cases because mutualdiffusion of Au and Sn is very rapid. With the structure shown in FIG.2, however, the Ag film 4 functions as a barrier for mutual diffusion ofAu and Sn to enable preventing diffusion of Au into Sn. Accordingly,surfaces of solder are not oxidized and an excellent wettability can beobtained.

In addition, the provision of an Au layer on an Ag layer is applicableto the following embodiments described below. In this case, when an Aglayer is formed by means of the thin film technique, an Au layer iscontinuously formed by means of thin film technique. When an Ag layer isformed by means of plating, an Au layer is continuously formed by meansof plating.

Embodiment 3

A still further embodiment for the substrate according to the firstembodiment of the invention will be described with reference to FIG. 3.Here, FIG. 3 is a cross sectional view showing a connection portion ofthe substrate.

The substrate 30 shown in FIG. 3 is formed such that a flexiblepolyimide foil 5 and a copper foil are caused to bond to each other, Snsolder bumps 7 are formed on a Cu electrode metallization layers 6,which is formed by patterning the copper foil, by electroplating, and anAg plated film 8 is formed on the Sn solder bumps 7. The composition ofthe solder bumps 7 may include an alloy solder described in themodification of the embodiment 1. In addition, a polyimide foil, whichis disposed on an upper side in the figure and sandwiches the copperfoil, is omitted for the sake of simplification of depiction.

When the Ag plated film 8 is formed to be thick, it covers the wholesurfaces of the solder bumps 7, but minute non-plated regions aregenerated by adjusting a thickness of the film appropriately and formingthe film to make the same a little thin. Since surfaces of the solderbumps 7 are slightly exposed in these regions, the effect ofantioxidization of the Ag plated film 8 is produced by the cell reactionin the wet process. Also, likewise, since the Ag plated film is notoxidized at 25° C. in the atmosphere, any oxide film is not formed in Agplated portions, so that any adverse influence is not made to theconnection property.

If the flexible substrate 30 of such configuration is used, connectionportions of electronic parts (not shown) being connected to thesubstrate 30 can be positioned in such a manner as to contact with theAg plated film 8 on the solder bumps 7, and the substrate 30 and theelectronic parts can be connected to each other in a fluxless manner byperforming reflow. Advantages of fluxless connection include reductionin cost for flux itself, reduction in cost owing to reduction in thesubsequent cleaning processes, a decrease in damage to parts by flux,etc.

Embodiment 4

An electronic part according to the second embodiment of the inventionwill be described with reference to FIG. 4. Here, FIG. 4 is a partial,cross sectional view showing a connection portion of the electronicpart.

The electronic part 40 shown in FIG. 4 is formed such that a circuitelement 70 is formed on a Si wafer 9, then a plating resist (not shown)is applied on metallization layers 11 at the Si wafer level, solderbumps 12 are formed by plating, and after peeling-off of the platingresist, an Ag film 13 is formed by means of mask sputtering.

As in the case of the embodiment 1, the solder bumps 12 can be formed bycarrying out metal/alloy plating of Sn, Sn—Ag, Sn—Ag—Cu, Sn—Zn, Sn—Pb,etc. Further, formation of the Ag film 13 makes it possible to preventoxidation of surfaces of the Ag film 13 caused by oxygen and moisture inthe atmosphere.

In addition, while after peeling-off of the plating resist, the Ag film13 was formed by means of mask sputtering in the embodiment, Ag platingmay be carried out consecutively after the solder bumps 12 are formed.In this case, Ag plating is somewhat different in shape from the Ag film13.

In recent years, wafer-level mounting of electronic parts has beenexamined, and so a technique of forming solder bumps on an electronicpart at the wafer level has become important. Since a wafer formed withsolder bumps is then divided in a dicing process, the solder bumps aresurely exposed to a cooling water in dicing. Accordingly, corrosion ofsolder bump surfaces caused by moisture causes an important problem.Corrosion of solder bump surfaces can be prevented by using thestructure according to the embodiment.

Embodiment 5

A further embodiment of an electronic part according to the secondembodiment of the invention will be described with reference to FIG. 5.Here, FIG. 5 is a cross sectional view showing the electronic part.

The electronic part 50 shown in FIG. 5 is formed such that asemiconductor chip 14 is mounted on a lead frame 15, the lead frame 15and the semiconductor chip 14 are connected to each other by bondingwires 18, and the whole is molded by a resin 19. A solder plated film 16and an Ag plated film 17 are formed on lead connection portions.According to the embodiment, the solder plated film 16 can be made bymetal/alloy such as Sn, Sn—Ag, Sn—Ag—Cu, Sn—Zn and Sn—Pb. The effect ofantioxidization of the solder plated film 16 produced by the Ag platedfilm 17 is the same as those in the embodiments described above.

In addition, while a substantially whole region of the solder platedfilm 16 is covered by the Ag plated film 17 in FIG. 5, it is necessaryto carry out Ag plating only on a horizontal portion (a portionconnected to a substrate) of leads of Small Outline Package so far asthe connection property is concerned. The horizontal portion of theleads is depicted as an end of each portion of the lead frame 15extended outside the resin (the resin mold) 19 in FIG. 5, and isextended horizontally. The substrate (not shown in FIG. 5) isexemplified as one of external members (external circuits) to theelectronic part 50. Also, function elements built in an electronic partare not limited to semiconductor chips (as the semiconductor chip 14shown in FIG. 5) but may be resistive elements, capacitor elements, etc.

The embodiment can provide an electronic part, in which a minute amountof solder plated on an electronic part enables mounting on a substratewithout oxidization of a solder surface and without the use of flux.

Also, according to the embodiment, the substantially whole region of thesolder plated film 16 is covered by the Ag plated film 17, so that thereis produced an effect of enabling prevention of generation of Snwhisker.

According to the invention, it is possible to provide an electronicpart, in which an antioxidization film for prevention of oxidization ofa surface of solder is formed on a solder provided on a connectingportion of the electronic part and connection can be achieved in afluxless manner.

1. An electronic part comprising: a base material; metallization layersformed on the base material; and a Sn solder portion formed on a part ofa surface of said metallization layers; wherein the electronic partfurther comprises an Ag film formed on a surface of the Sn solderportion and an Au film formed on the Ag film.
 2. The electronic part asclaimed in claim 1, wherein the Sn solder portion is composed of analloy solder containing Sn as a main component.
 3. The electronic partas claimed in claim 1, wherein the Ag film is composed of an alloycontaining Ag as a main component.
 4. The electronic part as claimed inclaim 1, wherein the Ag film is formed so that a ratio of Ag is 73 wt %or less based on a total amount of the Sn solder portion and the Agfilm.
 5. The electronic part as claimed in claim 1, wherein the Ag filmis provided as a barrier between the Sn solder portion and the Au filmpreventing diffusion of Au into Sn.
 6. An electronic part comprising: alead frame; a function element mounted on the lead frame; a plurality ofbonding wires which connect terminals of said function element and leadsof the lead frame; and a resin part molding the function element, theplurality of bonding wires and a part of the lead frame; wherein leadsextending outward from the resin part are subjected to Sn plating, an Agfilm is formed on connecting portions of the leads which are subjectedto Sn plating and an Au film is formed on the Ag film.
 7. The electronicpart as claimed in claim 6, wherein the Sn solder portion is composed ofan alloy solder containing Sn as a main component.
 8. The electronicpart as claimed in claim 6, wherein the Ag film is composed of an alloycontaining Ag as a main component.
 9. The electronic part as claimed inclaim 6, wherein the Ag film is formed so that a ratio of Ag is 73 wt %or less based on a total amount of the Sn solder portion and the Agfilm.
 10. The electronic part as claimed in claim 6, wherein the Ag filmis provided as a barrier between the Sn plating and the Au filmpreventing diffusion of Au into Sn.